This invention relates to a method for inspecting for evidence of leakage of a steam expansion vessel, and more particularly, to a method for inspecting for evidence of a pinhole leak in a steam expansion vessel.
Steam expansion vessels may be used in industry to activate equipment, and may have application for a variety of purposes.
Many curing devices for molded products use an expansion vessel, also known as a curing bladder. In a curing device, most expansion vessels are pressurized with a fluid heat source, such as steam. The steam causes the expansion vessel to expand, pushing the product tightly against the mold. Further, the heat from the steam helps to at least partially cure the molded product.
Problems occur when a defect, such as a pinhole leak, forms in the expansion vessel. In the tire industry, pinhole leaks in expansion vessels on curing devices are one of the industry""s largest causes of tire scrap. Pinhole leaks in expansion vessels cost tire manufacturers millions of dollars per year. A pinhole leak allows high temperature steam to contact the innerliner of the tire. One possible result of this contact is an undercured area in the innerliner. Another possible result is the permeation of steam into the ply area of the tire, causing a blister that can result in the separation of the plies. Currently, the first inspection of a tire for the effects of pinhole leaks occurs in the final finish area of the tire plant, minutes after the tire leaves the curing device. If a tire in final finish is found to be defective as a result of a pinhole leak, then up to ten additional tires may have been cured with the same defective expansion vessel prior to discovery of the problem. Generally, all of these tires must be scrapped.
In order to reduce scrap caused by defective expansion vessels, expansion vessels are changed after a set number of cycles. This preventative measure does not always prevent scrap, however, because a pinhole leak may develop prior to this set number of cycles. Additionally, this preventive measure may change out some expansion vessels that still have many more cycles in their effective life. Thus, scrap tires may still result, money is lost by not utilizing the full life of the expansion vessel, and additional labor costs arise due to more frequent expansion vessel changes.
This has been a continuing problem in the tire industry since bladder curing was introduced in the 1960""s. The industry has made many attempts to solve this problem with varying degrees of effectiveness.
Cole et al. in U.S. Pat. No. 3,942,922 teach an apparatus for curing a tire having an internal cavity, using steam as a heat and fluid pressure media. The time rate of pressure loss in the cavity is determined in each tire curing cycle by closing the cavity to form a completely closed chamber for a test time less than the normal cure time for a tire. Pressure loss during the test time is monitored and if determined to be greater than normal, a warning device is actuated and means for loading the next tire into the mold is rendered inactive.
Garrison, in U.S. Pat. No. 4,188,818 teaches a self-contained device for selectively discharging a pressurized gas within a hermetically sealed vessel to enable leak testing of the vessel. A linear actuator is used to open a small, high pressure cylinder that is charged with a detector gas. The mechanism is mounted on the interior of a lid that is sealed onto the vessel which is to be tested.
Jones, in U.S. Pat. No. 4,221,124 teaches a low-level flow sensor for detecting bladder leaks and ruptures. A dual mode logic circuit is used to monitor for bladder leaks where there are low level failures during a preselected portion of a curing cycle, and at other times requires sustained flow to the sensor for a predetermined time to indicate bladder ruptures or high-level failures. A warning signal is generated and the cycle is interrupted to prevent loading of a next tire into a curing press if a leak is detected.
Curing device manufacturers have attempted to limit scrap products caused by pinhole leaks in expansion vessels by incorporating a monitoring system into the curing device. Before a product to be cured is placed into the curing device, the expansion vessel of the curing device is pressurized. The pressurized expansion vessel is isolated by closing the valves in the supply and discharge lines. The monitoring system monitors the pressure in the expansion vessel to determine if a leak is present.
U.S. Pat. 5,417,900 entitled xe2x80x9cVacuum Leak Detector for a Tire Curing Pressxe2x80x9d discloses a device and a method for detecting a leak in an inflatable elastomeric bladder of a curing press. The spent curing fluid is pumped through an exhaust line in which a venturi ejector is located. The venturi ejector causes a vacuum to be created as the fluid is being removed. A vacuum sensor monitors this vacuum. If a predetermined vacuum strength is not reached in a predetermined time, a leak is assumed to exist.
These prior art systems work well when a leak occurs at the coupling attaching the expansion vessel to the curing device. However, these systems are not adequate for detecting a pinhole leak in an expansion vessel. Since the quantity of fluid lost through a pinhole leak is small, relative to a coupling leak, determining the presence of such a leak using pressure and/or vacuum sensors is very difficult. These devices also signal leaks in bladder seals.
The present inventors have long searched for an effective solution to the problem of detecting leaks in a steam expansion vessel. In the past, detection methods that were tried included sensing the mass flow of nitrogen from the mold; the addition of dye to cure steam and the use of a UV sensor or a color sensor to detect the escape of the dye; the use of tracer gases inserted into a bladder, and the use of gas sensors to detect the tracer gases; the use of infrared thermal sensors; and testing for relative humidity around the mold.
While all these methods theoretically could be used, it was found in application that each method had some practical limitations. In testing for relative humidity, for example, it was found that at high temperatures xe2x80x9crelative humidityxe2x80x9d has less meaning since water vapor does not condense easily, and the dew point is so high that normal means for testing for relative humidity are not applicable.
A method for detecting leaks in a steam activated expansion vessel is used in industrial equipment, and comprises the steps of creating or selecting a path in industrial equipment through which steam would travel if escaping from a steam activated expansion vessel, and placing a humidity sensor in the path.
The method of detecting leaks comprises the further step of selecting a humidity sensor that has the ability to detect moisture at 25xc2x0 C. to 200xc2x0 C., and comprises the step of using data from each activation cycle to establish an average or baseline for humidity of a plurality of activation cycles, and comprises the further step of providing computer control for the industrial equipment, such that when a leak is detected an alarm is sounded and the industrial equipment is automatically shut down.
A further step is provided of placing a filter in the path between the activated expansion vessel and the sensor, the filter being pervious to water vapor.
In the illustrated embodiment, the industrial equipment is a curing mold for a pneumatic tire, and the path includes a bolster screw in an area of the mold in the proximity of a bead area of a tire which is being cured in the mold, and the method comprises the further step of placing the humidity sensor in the bolster screw. The curing bladder is substantially centered on the tire being cured, and the bolster screw is in the proximity of a curing bladder center post.
In an alternative embodiment, a conduit can be run from the bolster screw to a position remote from the mold, and the humidity sensor can be placed at the end of the conduit.
The mold in the illustrated embodiment is a two piece mold, and the method may comprise the further step of placing the sensor in or around an upper portion of the mold.
An apparatus for curing an article comprises a shaping means for shaping the article during curing, an expansion vessel within the shaping means, the expansion vessel having the capacity to withstand elevated temperatures and high pressures. In the operation of the apparatus, the expansion vessel is inflated with steam and used to push the article into the shaping means, and an absolute humidity sensor is located in the proximity of the expansion vessel and the shaping means, in a path selected to detect leaking steam from the expansion vessel.
The apparatus for curing an article comprises a curing mold for a pneumatic tire, and an absolute humidity sensor which has the ability to detect moisture at 25xc2x0 C. to 200xc2x0 C. In the illustrated embodiment, the apparatus comprises a two-piece tire mold. The apparatus further comprises a magnet attached to the curing mold, the absolute humidity sensor being attached to the magnet, whereby the absolute humidity sensor is held in place by the magnet.
Definitions
For ease of understanding this disclosure, the following terms are defined.
xe2x80x9cBeadxe2x80x9d means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards, and chafers, to fit a design rim. The beads are associated with holding the tire to the wheel rim.
xe2x80x9cCuringxe2x80x9d means the process of heating or otherwise treating a rubber or plastic compound to convert it from a thermoplastic or fluid material into a solid, relatively heat-resistant state by causing cross-linking of the compound.
xe2x80x9cElastomerxe2x80x9d means a resilient material capable of recovering size and shape after deformation.
xe2x80x9cElastomeric articlexe2x80x9d is an article that is at least partially made from an elastomer.
xe2x80x9cInnerlinerxe2x80x9d means the layer or layers of elastomer or other material that form the inside surface of a tubeless tire and that contain the inflating fluid within the tire.
xe2x80x9cPneumatic tirexe2x80x9d means a laminated mechanical device of generally toroidal shape, usually an open torus, having beads and a tread and made of rubber, chemicals, fabric and steel or other materials. When mounted on the wheel of a motor vehicle, the tire, through its tread, provides traction and contains the fluid that sustains the vehicle load.
xe2x80x9cRadialxe2x80x9d and xe2x80x9cradiallyxe2x80x9d are used to mean directions toward or away from the axis of rotation of the tire.